1. Field of the Invention
The present invention relates to a computer program product, system, and method for synchronous mode replication to multiple clusters.
2. Description of the Related Art
A replication manager replicates data to multiple different clusters so that copies of the data are maintained at multiple sites and updated to provide consistent and synchronized copies of the data at the different cluster sites. In a virtual tape library (VTL) environment, the clusters to which host data is replicated may comprise virtual tape libraries that store tape formatted data on a non-tape storage, such as disk, that emulates tape with a faster access profile. VTLs may also be used with physical tape providing backup storage of the VTLs. Policy settings indicate how the data is to be replicated to the clusters. Two common replication settings for replicating host data to VTL clusters include immediate copy and deferred copy.
With the deferred copy mode, after the host job/application finishes with a volume, a copy of the data (logical volume) is queued for later execution, such as after the issuance of the rewind/unload (RUN) command during demount processing, which may occur minutes to hours after being queued. With the immediate copy mode, after the host job/application finishes with a volume, a copy of the volume to a remote location is made while the library holds device-end on the rewind/unload (RUN) command. In this case, device-end is held until the copy completes to a remote location, which could be several minutes.
The above deferred and immediate copy modes are performed at a cluster transparent to the host operations. Another technique for replicating host writes to multiple clusters and ensure a Recovery Point Objective (RPO) of zero, which means no risk of losing data, is for the host to perform a host process of tape duplexing, where the host write does not complete until the data is confirmed copied to the two clusters. Host tape duplexing provides data protection at synchronize command granularity (as the data was being written) with two tapes in the same or different library being simultaneously written to (the primary and the alternate). Host tape duplexing may require manual intervention if the alternate copy is needed and may introduce overhead in the host application because the host must perform the duplexing and tracking of the two duplexed copies.
Host tape duplexing is implemented in the International Business Machines Corp. (IBM) z/OS DFSMShsm product. (z/OS and IBM are trademarks of International Business Machines Corp. in the United States and other countries).
There is a need in the art for improved techniques for replicating host writes to cluster sites.